by Fran Moriarty

Created on: April 13, 2009

How did dinosaurs disappear? The fossil record portrays dinosaurs as a highly successful animal during the Mesozoic Era (250 million to 65 million years ago). It is not unusual for living things to become extinct. In fact some estimates claim that the presently living life forms represents only 1% of all the life forms that has existed. The other 99% are extinct. Dinosaurs died off relatively quickly and the evidence seems to indicate a dramatic exit.

Basically there are four main accounts that explain the extinction of dinosaurs. They are, In order of most popularity: diastrophism theories, cyclic theories, Cope's Rule and dino-disease. All the theories offered can fit into one of these categories.

The diastrophism theories hypothesize that some spectacular event triggered catastrophic events causing rapid changes in the environment. Volcanoes erupting poisonous gases and sun-blocking debris, tidal waves and tsunamis, earthquakes, and temperature declines contributed to the a new set of survival challenges. Only the species better suited for this harsh environment survived to reproduce. The difference in diastrophism theories is in terms of what triggered the cataclysm. There is the Asteroid Impact Theory, Meteorite Extinction Theory , Supernova Theory and finally the Volcanic Theory. The evidence supporting the Asteroid Impact Theory is strong and most new findings are supporting that explanation.

The cyclic theories refer to some event that occurs on somewhat of a regular basis that causes major shifts in climate and may cause flooding and possible volcanic activity. The Nemesis Hypothesis proposes that an "Oort cloud" appears within the area of earth every 26 million years. Within the "Oort cloud" are comets and space debris that bombard the earth. The Orbital Shift Theory refers to changes that occur in the earth's orbit around the sun while the Axis or Wobble Theories refer to regular shift of the polar axis. The only evidence for the Nemesis Theory is the apparent higher extinction periods that occur about 26 million years apart. Over the 3 billion years of life on earth, there have been five major mass extinctions. The evidence is weak at best. The global shift and wobble theories are feasible but questionable if it would account for mass extinctions. Present day fears of Ice Age temperatures and global warming may be outgrowths of these accounts.

Cope's Rule hypothesizes that living things evolve into larger life forms over time. If conditions are ripe,

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